1. Field of the Invention
This invention relates, in general, to photocopying and, more specifically, to apparatus which scans documents and converts the scanned signals into binary data.
2. Description of the Prior Art
Electronic copy machines, and other similar devices, can use an electronic scanner to develop digital information relative to the content or image patterns of an original document. Many scanners are capable of scanning the original and providing the output data in a multi-bit digital format. With such data, the gray scale or degree of density of a particular scanned pixel can be represented by the value of the pixel data. Although this form of data is useful in some applications, other applications only need the data in one-bit, two-valued, binary form for each pixel. There can be many reasons for this requirement, including memory conservation, lack of gray scale information in the second document, and the inability of the reproducing apparatus or print engine to produce multi-bit, gray level, image information.
A well known process for converting digital information into binary information involves the use of a threshold level, or value above and below which different binary values are specified. While this type of conversion system is easy to implement, it has certain limitations and drawbacks when used to convert scanned information containing high contrast text characters and graphic lines. These limitations are inherent in the operation of such scanners because of the modulation transfer function (MTF) of the scanner and the inability of the scanning to be exactly in phase with the boundary or borders of the text characters. The result is a void of pixels along the text boundaries, sometimes amounting to several pixels in a row. Because of the large voids or gaps produced along the edges of the characters using this technique, the voids are noticeable and the characters do not appear to have precisely defined shapes. Conventional thresholding also has limitations when processing continuous and halftone pictorial information since much of the medium density data is lost in the conversion process to only two levels.
In order to overcome some of the limitations of the prior art conversion systems, several techniques have been used with differing degrees of success. One system uses what is known as adaptive thresholding wherein the value or level of the threshold changes during the processing, depending upon some function of the values being processed. Other forms of conversion systems known in the prior art use edge enhancing and noise cleaning to reshape the edges of characters. However, these systems require a knowledge of the shape of the characters which the pixels represent. This adds the limitation that this type of processing can only be used for known character shapes. Random shapes or nonstandard characters are not easily enhanced using such techniques.
U.S. Pat. No. 4,466,123, issued on Aug. 14, 1984, teaches a processing system for correcting contour line pattern images. In this system, the data being processed is in binary form, and the patent does not disclose any system for purposely introducing an alternating pixel pattern along the edges of the characters, as does the present invention. U.S. Pat. No. 4,517,604, issued on May 14, 1985, teaches a method for reducing line width variations in bilevel video images whereby a gray level pixel or pel is added to the ends of certain lengths of black pixel scans. Unlike the present invention, this patent describes a technique wherein the treatment at the edges of a black line scan is dependent upon the number of black pixels includes in the line scan.
U.S. Pat. No. 4,544,264, issued on Oct. 1, 1985, is another patent which teaches a form of line enhancement which has been described in the prior art but which is different than that disclosed herein. In that patent, a systematic addition or deletion of gray pels or pixels at the end of a scan line is determined by the number of black pels in a row, primarily lines of a single pixel in width. The gray pels are added to make the line thicker and the addition is dependent upon the direction of the scan.
Therefore, it is desirable, and it is an object of this invention, to provide a system for processing digital, gray scale data into binary data which is convenient to implement, and which can be used on ccharacters of unknown format and on images and graphics of other types.